While the drilling of wells for the production of hydrocarbons, such as oil and natural gas, has always been quite expensive, even more attention has been paid to drilling costs in recent years. This is due in part to the increasing depth and difficulty of location of remaining hydrocarbon reserves, considering that many shallow and large reservoirs have already been heavily exploited. As drilling costs increase at least linearly with the depth of the well being drilled, newer wells are becoming increasingly expensive. Drilling in hostile surface or sub-surface environments increases the drilling costs. Furthermore, the volatility of prices in the oil and gas markets in recent years has reduced operating profit margins, and thus has placed significant pressure on producers to drill only where the likelihood of paying production is high.
Faster and more efficient drilling, in distance drilled per unit time, is of course highly desired to contain these costs. However, overpressurized sub-surface zones present significant problems to drilling in many locations, as drilling into such a zone causes a blow-out if the pressure of the hydrocarbon (generally natural gas) in the zone exceeds the pressure in the wellbore to such an extent that the hydrocarbon explodes out of the well. In locations where overpressurized zones are expected, drilling must be performed using heavy drilling mud to increase the pressure in the wellbore to hold the hydrocarbon in the overpressurized zone in place when the zone is reached. As is well known in the art, however, drilling with such heavier muds is significantly slower than drilling with lighter muds. Due to the limited accuracy with which conventional seismic surveys can predict the depth of such zones, heavy mud is used over relatively long distances to provide sufficient safety margin. As a result, drilling efficiency is significantly impacted by such conventional drilling and exploration techniques.
In addition, while the use of heavy muds reduces the likelihood of a blow-out, excessively heavy mud used during drilling can damage surrounding formations if the mud pressure is significantly greater than the so-called pore pressure in the earth. Therefore, the weight of the drilling mud has both an upper and a lower limit, outside of which drilling failure can occur.
Inaccuracies in the conventional surface geophysical surveys of course also add uncertainty to the success of the well in reaching any hydrocarbon reservoir. Particularly in many regions of the earth where exploration is currently taking place, reservoirs are limited in size, or may have a narrow cross-section in the plan view. A well drilled according to a conventional survey may narrowly miss the reservoir, where small deviations in the drilling direction would have resulted in success for the well.
For these and other reasons, it is therefore beneficial to acquire accurate information about the physical properties of the formations being drilled during the drilling operation, particularly concerning formations which are ahead of the drill bit. Such information can supplement that which was previously acquired by conventional surface geophysical surveys, and allow for control of the drilling to adjust for any differences between previously acquired information and the actual formations encountered. Furthermore, it is also beneficial to acquire accurate real-time information concerning certain drilling parameters, such as weight-on-bit, RPM, direction of the drill bit, and the like. This information, particularly in combination with surface survey information and information acquired during the drilling about the formations through which drilling has taken place and also into which drilling is about to take place, can allow for intelligent drilling, with parameters modified and adjusted on a real-time basis for maximum efficiency and the highest chances for successful production therefrom. The ability to acquire and utilize this type of real-time data is the goal of the invention described hereinbelow.
By way of background relative to the current state of the art, one type of exploration while drilling method which is known in the art is the "TOMEX" method presently offered by Western Atlas International, Inc. According to this method, energy imparted into the earth by the drill bit, during the drilling operation, is considered as the source energy for seismic surveying, with reflections of this source energy detected by geophones deployed at surface locations away from the drilling location. The "TOMEX" survey method is described in numerous publications, including Rector III, et al., "Extending VSP to 3-D and MWD: Using the drill bit as downhole seismic source", Oil and Gas Journal, (Jun. 19, 1989), pp. 55-58, and in Rector, Marion and Widrow, "Use of Drill-Bit Energy as a Downhole Seismic Source", 58th International Meeting of SEG, paper DEV 2.7, pp. 161-164, U.S. Pat. Nos. 4,363,112 and 4,365,322, and PCT publication WO 88/04435.
However, certain limitations are believed to be present relative to the use of downhole seismic sources in conjunction with surface receivers, such as in the "TOMEX" survey method. Firstly, due to the distance traveled by the seismic energy through the earth, only relatively low frequency (and long wavelength) energy is useful. As a result, the resolution of such surveys is necessarily limited. Secondly, it is quite difficult to obtain an accurate source signature, or pilot signal, from vibrations transmitted along the drill string from the bit to the surface. For example, in the "TOMEX" survey where the source signal is detected by monitoring drill string vibrations, noise of significant amplitude couples into the source vibrations detected at the surface, making determination of the source signature (for purposes of later correlation with the geophone-detected vibrations) difficult and inaccurate. Such difficulties with the noise in drill string vibrations are described in copending application Ser. No. 564,621, filed Aug. 8, 1990, assigned to Atlantic Richfield Company and incorporated herein by reference, in J. P. DiSiena et al., "VSP While Drilling: Evaluation of TOMEX", Exploration Technology Reiort (Atlantic Richfield Company, Fall 1989), pp. 13-20, and also in U.S. Pat. No. 4,954,998.
By way of further background, other known analysis methods utilize energy that is generated downhole (for example by the drill bit) and detected at the surface, besides that described hereinabove for seismic surveying. For example, the vibrations in the drill string which are generated by the interaction of the drill bit with the formation can be detected at the surface and analyzed to provide real-time monitoring of drilling conditions and parameters. U.S. Pat. No. 4,715,451, issued Dec. 29, 1987, assigned to Atlantic Richfield Company and incorporated herein by reference, describes a method and system for monitoring drilling parameters by way of spaced apart subs at the upper end of the drill string, such subs including accelerometers and strain gauges. The monitored parameters include axial and torsional loading on the drill bit, axial and torsional drillstring vibrations, and bending modes of the drillstring.
By way of further background, other "measurement-while-drilling", or "MWD", techniques utilize downhole sensors of various parameters, in combination with one of several approaches for telemetry of the detected parameters. Various examples of such approaches are described in Honeybourne, "Measurement While Drilling", Symposium on the 75th Anniversary of the Oil Technology Course at the Royal School of Mines (1988), particularly relative to mud pulse telemetry.
U.S. Pat. No. 4,992,997, issued Feb. 12, 1991, assigned to Atlantic Richfield Company and incorporated herein by reference, describes a stress wave telemetry system for monitoring downhole conditions during drillstem testing, or during wellbore stimulation or fracturing; this system includes accelerometers or strain gauges mounted onto the drillstem near the surface, for sensing torsional, axial or bending vibrations in the drillstem which may be correlated to downhole conditions. The above-referenced related copending applications Ser. No. 554,022 and 554,030, both filed Jul. 16, 1990, both assigned to Atlantic Richfield Company, and both incorporated herein by this reference, describe another example of a telemetry system, where a transducer disposed within the drill string provides high data rate telemetry from downhole to the surface by way of acoustic axial or torsional vibrations. These techniques communicate data on a real-time basis, without requiring that drilling be stopped as in the case of conventional well log tools and techniques.
By way of additional background, conventional wireline logging tools are used to evaluate the properties of formations surrounding wellbores, in conjunction with drilling operations. These logging tools are lowered into the wellbore periodically in the operation, with the actual drilling and excavation stopping during the logging operation. These downhole logging tools include radioactive and electromagnetic instrumentation, of various types.
A first type of electromagnetic logging tool is the direct coupled, or galvanic, logging tool. An example of a currently available galvanic logging tool are those of the well-known "Laterolog" type, available from Schlumberger. Such galvanic logging tools source a current into the earth from one electrode, for example the upper portion of the drill string, and measure a potential difference with other electrodes in the logging tool. Conventional galvanic logging tools have a relatively shallow depth of investigation (on the order of inches to several feet), as the information of interest is the resistivity of the formation immediately outside of the so-called invaded zone; accordingly, the distance between a potential-measuring electrode and one of the current electrodes is quite small. Logging tools of the Laterolog type include an opposing current, to focus the investigation into the formation within a narrow plane perpendicular to the borehole. The Laterolog principles are also used in "measurement-while-drilling" galvanic tools, such as available as the "FCR" measurement system from EXLOG.
The second type of electromagnetic wireline logging tool is often referred to as an electromagnetic induction tool. In this tool, two coils are lowered into the wellbore, separated along the axial length of the wellbore. One of the coils is energized to produce electromagnetic waves of known frequency and amplitude, and the other coil measures the electromagnetic energy it receives from the first coil, after the waves have traveled through the formation. Analysis of the amplitude attenuation and phase shift of the received waves from the transmitted waves will be indicative of the impedance of the surrounding formation.
In the case of these induction tools, it should be noted that the measurement is directed substantially perpendicular to the axis of the wellbore (at the location of the tool), but only for a limited distance. This is due to the purpose of this tool of determining the local resistivity of the surrounding formation, assuming homogeneity of the formation. The distance of interest from the wellbore is preferably far enough away so that the effects of drilling mud packing into the near-wellbore layer of the formation are minimized, but not far enough away that another formation type is encountered by the waves. Since the logging by this tool assumes (and relies upon) homogeneity of the measured layer, the readings and analysis of the received energy from multiple formation types is undesired. Typical distances over which the waves of interest travel are on the order of 10 feet from the wellbore, in substantially a perpendicular plane therefrom.
"Logging-while-drilling" tools, which provide surrounding formation analysis by monitoring certain types of radioactivity (such radioactive measurements conventional for wireline logging tools) and which apparently may be used during drilling, are known to have been developed by Magnetic Pulse, Inc. The measurements available from this tool include the passive measurements of gamma ray emission from the surrounding formation, including spectral analysis of the gamma ray emission to determine the presence of certain elements in the formation. The tool is also apparently capable of neutron density measurements, as the tool has a neutron source (such as AmBe) and detector, such that the density of the formation can be determined by the number of neutrons detected after back-scattering by the formation to the neutron detector. A Cesium gamma ray source in such a tool is also known, such that density measurements may also be made by detecting gamma ray back-scatter from the formation.
By way of further background, Bradley, et al., "Microprocessor-Based Data-Acquisition System for a Borehole Radar", IEEE Trans. Geoscience & Remote Sensing, Vol. GE-25, No. 4 (IEEE, 1987) describes the use of a downhole radar tool for evaluating the formations surrounding the wellbore. By way of still further background, van Popta et al., "Use of Borehole Gravimetry for Reservoir Characterisation and Fluid Saturation Monitoring", Publication 988 (Shell Internationale Research Maaschappij B. V., 1990) describes a method of measuring secondary gas saturations in a fractured reservoir using borehole gravimetry.
By way of further background, U.S. Pat. Nos. 4,929,896, 4,906,928, 4,843,319, 4,839,593 and 4,929,898, all assigned to Atlantic Richfield Company and all incorporated herein by reference, describe systems for the measurement of the thickness of a conductive container, such as a pipe, by way of current induction. This method is commonly referred to as transient electromagnetic probing, or "TEMP". In these systems, a transmitting antenna generates a magnetic field, which in turn produces eddy currents in the conductive container being measured. These eddy currents produce a magnetic field, which is measured by a receiving antenna. The rate of decay of the measured current corresponds to the rate of decay of the eddy currents in the container being measured, which corresponds to the thickness of the conductive walls or coating of the container. Accordingly, these systems allow for non-contact measurement of the thickness of containers such as petroleum pipelines, so that the effects of corrosion may be monitored.
It is an object of this invention to provide a method and system for obtaining accurate seismic data, with high spatial resolution, which looks ahead of the drill bit during the drilling operation into nearby formations.
It is a further object of this invention to provide such a method and system which utilizes acoustic vibrations generated by the drill bit as the source for such data.
It is a further object of this invention to provide such a method and system which utilizes electromagnetic energy, both DC and induction, generated downhole.
It is a further object of this invention to provide such a method and system which can detect approaching overpressurized zones, so that drilling efficiency may be maximized by use of heavier drilling muds only in those regions at and near such overpressurized zones.
It is a further object of this invention to provide such a method and system which can provide for optimized casing design, relative to the heavy weight mud which must be used once such an overpressurized zone is reached.
It is a further object of this invention to provide such a method and system which includes downhole sensors of downhole-generated source energy, to provide for improved accuracy in the resulting data analysis.
It is a further object of this invention to provide such a method and system which includes high data rate telemetry for communication of the downhole sensed energy, to provide improved resolution look-ahead analysis.
It is a further object of this invention to provide such a method and system which includes downhole computing capability sufficient to provide real-time analysis of the downhole-sensed information, such that the results of the analysis can be communicated to the surface with even relatively low data rate telemetry.
It is a further object of this invention to provide such a method and system which utilizes downhole computing capability of sufficient performance as to allow conventional low data rate downhole-to-surface telemetry to communicate the results.
It is a further object of this invention to provide such a method and system which includes spaced apart downhole sensors for purposes of reduction of noise, and so that the resulting analysis can determine the location of certain sub-surface structures.
It is a further object of this invention to provide such a method and system which can provide information regarding the temperature and pressure near the bottom of the wellbore at data rates high enough so that pressure dynamics of flow and reservoir recovery can be used to assist in characterization of the reservoir.
It is a further object of this invention to provide such a method and system which can provide information regarding wellbore pressure, time rate of change of pressure, and pH of the surrounding fluid, in order to monitor the progress of acid treatment completion of oil and gas wells, and in order to monitor the extent of formation fracturing in completing oil and gas wells.
It is a further object of this invention to provide such a method and system which provides real-time drilling parameter monitoring capability in an improved manner.
It is a further object of this invention to provide such a method and system which can detect the presence of faults and interfaces which are at angles other than perpendicular to the direction of drilling.
It is a further object of this invention to provide such a method and system which can monitor parameters of formations through which drilling has already taken place, and to use this monitored information in providing a survey relative to formations into which drilling has not yet taken place.
It is a further object of this invention to provide such a method and system which utilizes spaced apart detection locations along the drill string so that drillstring interaction and distributed operator response characteristics can be measured.
Other objects and advantages of this invention will be apparent to those of ordinary skill having reference to the following specification together with the claims.